Achieving unexpected strength and ductility synergies in heterogeneous metastable lamellar steels

High-strength steel with excellent ductility is pivotal for the formability and safety of critical structural components. Here, a heterogeneous metastable lamellar steel, composed of alternating lamellar ferrite and austenite aligned with the rolling direction, was developed through an innovative combination of warm rolling and immediate annealing processes. This novel design overcomes the strength-ductility trade-off, achieving high ultimate tensile strength (∼1.2 GPa) and excellent uniform elongation (∼78 %), pushing the product of ultimate tensile strength and uniform elongation to an ultra-high level (> 90 GPa %). The high tensile strength is attributed to ultrafine lamellar grains and significant work hardening induced by the hetero-deformation and transformation-induced plasticity (TRIP) effect. The exceptional ductility is a result of the synergy of multiple plasticity mechanisms, including (i) the inherent plastic deformation ability of lamellar microstructure and the hetero-deformation-induced hardening in the early deformation period, (ii) the persistent TRIP effect induced by the lamellar austenite with high mechanical stability and the elimination of strain localization caused by prolonged strain hardening due to the coordinated deformation of lamellar austenite and ferrite in the middle deformation period, and (iii) delamination cracking in the late deformation period. This approach adopted in current work offers a straightforward and economically feasible pathway for fabricating advanced high-strength steel with superior performance.